drm/ast: Remove device POSTing and config from chip detection

There's way too much going on in ast_detect_chip(). Move the POST
and config code from the top of the function into the caller. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-6-tzimmermann@suse.de